508 ALTERATIONS IN THE HEIGHT OF TIDE-WATER 



Changes in the height to which tides ascend a stream 

 and produce sensible effects are not uncommon in navi 

 gable rivers, though they are more usually attended or 

 caused by a lowering of the level of the water in the upper 

 parts of the rivers themselves. If the bottom through 

 which a river flows be such that the rush of water is con 

 stantly scouring it out, so as to lower the natural level of 

 the river-surface, then the same height of wave entering 

 its mouth will ascend farther, or make its influence felt 

 higher up. This may be the case with the Hudson. Or 

 the mouth of the river may be widening, so that a larger 

 body of sea flows in ; and, being driven forward by the 

 rising wave, is forced to rise higher as it ascends the con 

 fined channel, both elevating the high-water surface, and 

 making the tide observable to a greater distance upwards. 

 Or, lastly, the increase of obstructions at some point 

 between Albany and the mouth of the river, having the 

 effect of damming back the river and preventing the ebb 

 tide from fully flowing away, would at once elevate the 

 permanent level at which the upper part of the river 

 stands the level of high water and the distance upwards 

 to which the tides are felt. 



A remarkable illustration of the converse of this latter 

 mode of action has been afforded by the river Wear 

 between Sunderland and Durham. The improvement 

 of the harbour at Sunderland, the clearing of the out 

 falls, and the dredging of the river for two miles above 

 its mouth, had, in 1842, caused a permanent lowering^ or 

 scouring out of the bed of the river of 5 feet 3 inches 

 below the level of its bottom in 1737, at a distance of 

 11 J miles from its mouth, (New Bridge,) while the level 

 of high water at the same place is permanently lowered 

 5 feet 7 inches. At Biddick, which is nine miles from the 

 mouth of the river, a the level of low water at spring 

 tides in 1737 was nearly the same as that of high water 

 at spring-tides in 1842.&quot; This very remarkable circum- 



